A communication system of the type with which the present invention is concerned consists of a plurality of units termed end units (EUs) and distribution units (DUs) connected together to form a switching network. Digital data messages are sent between the units by means of the network. (For convenience, we will here use the term "packet" or "data message" for messages conveying data between users, and the term "message" or "control message" for control messages concerned only with the internal management of the system.) In particular, we are concerned with communication using a network service that is connectionless; that is, in which each packet sent between any pair of units is treated independently of any other such packet by the network.
One distinction between an end unit and a distribution unit is that an end unit only forwards packets onwards into the network that it itself has originated. A distribution unit forwards both packets that is originates and packets that it receives from other units. Typically an end unit has only one link connecting it to the network, while a distribution unit has many links. The end units and the distribution units correspond respectively to the end systems and intermediate systems of an ISO standard (9542), but it will be realize that the present principles are not limited to systems conforming to that standard.
The operations of a communication system can generally be separated into a number of layers--the standard layer model is the 7-layer model of an ISO standard (7498). The present principles are not confined to that model, but it is convenient to use its terminology. We are primarily concerned here with layer 3, the network layer, which is concerned with the transfer of packets between units. Every unit has an identification at layer 3 which is unique in the whole system; for convenience, we can use the ISO standard term NSAP (network service access point) for this identification.